Ruger SR9C confusion

[ArmednReady]

So I take my SR9C out at least once a month to go to the range I usually put 100 rounds or so through it all n all id say I have over 600 rounds through it. I usually clean it after every shootin session..anyway to the point the other day I go to take it out to get it ready for the range and I notice there is what looks to be rust on the back of the mags. It was being stored in the original box from ruger I have 3 10 round mags for it, the mag that was in the pistol was rust free but the other 2 had rust in nearly the exact same spots how the hell is this possible? the gun has only been stored for a month at most Have any other SR9C owners had any similar issues? or any other SA gun owners had this problem? Its pretty confusing because I stored my P345 in the same fashion and never had this problem.....help! lol

[sniper4usmc]

light coat of CLP will prevent rust